In many control applications, it is often necessary to sense the speed of a rotating body. Some applications require installation of a speed sensor in harsh environments in which the sensor might be exposed to debris or other material that could damage the sensor or prevent it from operating. For example, it might be desirable in some applications to sense the speed of an engine cooling fan or other moving device on an internal combustion engine. One method of sensing the cooling fan speed might use a sensor that is mounted adjacent to the fan blades to sense the number of fan blades that pass the sensor over a given time period. The signal generated by the sensor could then be used to calculate fan speed. However, such a sensor would be exposed to the engine compartment environment and would be exposed to dirt or other debris generated by fan air flow that could interfere with the sensor operation or damage the sensor.
An engine cooling fan speed sensor would be particularly useful for engine controls or for cooling controls that optimize engine performance and environmental emissions, or otherwise require an engine cooling fan speed feedback signal. Variable speed drives for engine cooling fans are known in the art. One such system is disclosed in U.S. Pat. No. 4,899,861, which permits a control system to control engine cooling fan speed over a range of speeds. Using a fan speed sensor in connection with such a drive would enable the control to receive a fan speed feedback signal to more precisely control engine cooling fan speed.
Some of the same problems associated with using a speed sensor to sense fan speed on an internal combustion engine are present when applying a speed sensor to other severe environments. For example, on a track laying work vehicle such as a track-type tractor, it might be desirable to sense the rotational speed of a drive sprocket or an idler sprocket. It would theoretically be possible to place a sensor adjacent to the sprocket to sense the number of teeth passing the sensor. However, the sensor would be exposed to rocks and other debris that could interfere with the performance of the sensor or damage the sensor. It would therefore be preferable to have a sensor that can sense the rotational speed of a rotating body, but at the same time, be protected from the operating environment of the rotating body.
In still other applications, it is sometimes necessary to mount a speed sensor at one location on an object and have the wires connected to the sensor come out of the object at a second, remote location. In these applications, it is often necessary to route electrical conductors through the object. This often entails first drilling or otherwise machining a hole through the object. Then, the speed sensor is installed and the wiring routed through the machined hole. It is difficult and labor intensive to manually route the wires through the hole. Moreover, the wires are also susceptible to damage as they are threaded through the hole. It would be desirable to have a speed sensor that could reliably sense the speed of a rotating body at some distance from the location where the electrical conductors emerge from the object.
Some manufacturers have recognized these problems and attempted to overcome them by installing a speed sensor in the shaft around which the rotating body turns. Since the sensor is installed within the shaft, it is not exposed to the environment of the rotating body. Those manufacturers have generally permanently installed a hall effect sensor in a hole drilled in the shaft. Wires are connected to the hall effect sensor. The wires extend through the hole, out an end of the shaft and are connected to an electronic control. The hall effect sensor and the wiring are permanently installed within the shaft using an epoxy or other similar substance. A magnet is placed on the rotating body, in a location that passes adjacent to the hall effect sensor when the body rotates about the shaft. As the magnet passes adjacent to the hall sensor, the hall sensor develops a signal. While this design works in some instances, the wires connected to the hall effect sensor, and the hall effect sensor itself, are susceptible to damage during installation. During installation the wire can be easily crimped, cut or otherwise damaged as they are manually threaded through the hole. This is especially true since the wires may be exposed to corners or other edges of the freshly machined metal. The exposed wire could then be grounded to the shaft, which would prevent the sensor from operating correctly. Since it is difficult to test the sensor without fully assembling the shaft and rotating body combination, the assembler would not be able to determine that the sensor had malfunctioned until after the sensor has been permanently installed in the shaft. At that point, if the sensor or wiring has failed, the whole shaft and rotating body must be remanufactured. The reliability and serviceability of such sensor systems are generally unsatisfactory.
An engine fan speed sensor of the form previously described has been developed by Rockford Powertrain, located in Rockford, Ill. That fan speed sensor includes a hall sensor that is an integral part of a shaft in the fan clutch. The hall sensor, circuitry and wiring are permanently fixed within the shaft during the manufacturing process. That process includes machining a channel in the shaft, threading connection wires through the channel, connecting the wires to the hall sensor, and permanently fixing the hall sensor and wiring within the channel using an epoxy or other similar substance. The process is labor intensive and difficult to accomplish without damaging the wiring or sensor. Furthermore, once the sensor and wires are permanently installed in the shaft, assembly of the fan clutch is more complicated and the sensor and wires are exposed to damage, which would then render the shaft useless. Because of the difficult, labor intensive assembly process, there are problems with the speed sensor's reliability. And, because the speed sensor is an integral part of the fan clutch shaft, the sensor cannot be individually replaced once it is found to be faulty or to have failed. Thus, the entire fan clutch must be removed from the engine and remanufactured making the serviceability of the sensor very low. For these and other reasons, it would be desirable to have a reliable, serviceable fan clutch speed sensor. It would also be preferable to have a replaceable modular speed sensor that overcomes the drawbacks and disadvantages associated with prior art speed sensor systems.